1. Field of the Invention
The present invention relates generally to function generators, and more particularly to a flexible transfer function generator.
2. Related Art
Power amplifiers are commonly used in a number of contemporary devices, including, for example, wireless communication devices such as handsets and basestations. In such devices, the power amplifier amplifies the modulated signal to a desired level such that the radiated power from the device's antenna is of sufficient strength to fall within the dynamic range of the intended receiver.
In many applications, closed loop systems are used to control radiated power to within a relatively narrow range. Closed loop systems typically accomplish this goal through the use of a feedback loop. Such feedback loops can include, for example, a power sensor at or near the antenna to detect the actual transmitted power level. The detected power level can be compared with a desired power level to determine a difference between actual and desired power levels. This difference, often represented as a voltage level, is integrated and fed back to the power amplifier to control the output of the power amplifier.
One drawback of contemporary closed-loop systems is that the power amplifier typically has a non-linear transfer function. As such, the response of the overall system, even with the feedback loop, is non-linear. This can result in less-than-desirable response characteristics of the feedback loop. For example, if the loop is operating in a region where there is too little gain, loop response may be slower than desired. If on the other hand, the loop is operating in a region where there is too much gain, the system may go into oscillation.
In some applications, the problem is more prevalent because the power amplifier tends to primarily operate in the non-linear region. For example, in devices such as wireless devices operating according to the GSM standard, the supply voltages tend to be very low and the output power is typically quite high. As such, in this type of application, the power amplifier often operates in the saturation region. As a result, the transfer function of the power amplifier exhibits the above-described non-linear characteristics.